The early diversification of ray-finned fishes (Actinopterygii): hypotheses, challenges and future prospects

Actinopterygii are the most speciose living vertebrate clade, and study of fossil members during their Palaeozoic rise to dominance has a long history of descriptive work. Although research interest into Palaeozoic actinopterygians has increased in recent years, broader patterns of diversity and diversity dynamics remain critically understudied. Past studies have investigated macroevolutionary trends in Palaeozoic actinopterygians in a piecemeal fashion, variably using existing compendia of vertebrates or literature-based searches, and there is no comprehensive occurrence-based dataset of actinopterygians spanning the whole of the Palaeozoic. Past studies typically show low levels of diversity in the Devonian with a substantial rise in the early Carboniferous in the aftermath of the end-Devonian mass extinction. However there are unresolved patterns reported for the later Carboniferous and Permian. In large part, these conflicts span from a lack of publicly-available occurrence data: actinopterygians are majorly underrepresented in the Paleobiology Database (PBDB), for example, obscuring patterns of diversity through time. This is exacerbated by major taxonomic problems pervading the Palaeozoic actinopterygian record. Innumerable taxa are lumped into wide-ranging families and poorly-formulated genera, with a vast number of described species concentrated in several particularly problematic ‘waste-basket’ genera. This taxonomic confusion feeds into a limited understanding of phylogenetic relationships. There is also a heavy sampling bias towards Europe and North America, with other regions underrepresented despite yielding important occurrences. Scrutiny of the extent to which spatial biases influence the record is lacking, as is research on other forms of bias. Low richness in some time periods may be linked to geological biases, while the effect of taphonomic biases on Palaeozoic actinopterygians have not yet been investigated. Efforts are already underway to both redescribe poorly defined taxa and describe taxa from underrepresented regions, helping address taxonomic issues and accuracy of occurrence data. New methods of sampling standardisation utilising up-to-date occurrence databases will be critical in teasing apart biological changes in diversity from those resulting from bias. Lastly, continued phylogenetic work will enable the use of phylogenetic comparative methods to elucidate the origins of actinopterygian biogeography and subsequent patterns of radiation throughout their rise to dominate aquatic faunas.